Concrete cutting saws, often referred to in the industry as slab saws, are used for cutting concrete and bituminous structures such as roadways and airport runways. As an example, when large slabs of concrete are poured, the surface of the slab is cut to divide the slab into smaller sections to relieve stresses and to allow for some degree of expansion and contraction which may otherwise undesirably crack the slab. Slab saws are also used to cut the edges of a portion of a slab that is to be removed. Additionally, such saws may be used inside of a building to perform similar functions on concrete floors.
Slab saws include a frame or housing typically mounted on wheels. Subject to the practicalities of saw size, a large, high horsepower reciprocating piston engine such as a Wisconsin 65HP engine has typically been used in concrete saws due to their relatively high torque for a given horsepower rating. Automobile engines such as a V6 engine have also been used to power large slab saws, but the resulting size of this saw has limited its utility and acceptance. However, reciprocating engines have a very steep torque curve in that the torque produced by the engine typically falls substantially as RPM decreases or increases around a relatively narrow ideal range. The saw includes a large diameter diamond tipped masonry cutting saw blade. When the saw is started, the operator positions the saw along a guide line, such as a chalk line, and lowers the saw blade into the concrete. As the saw blade cuts through the concrete the slab saw advances guided by the operator following the chalk line. A lubricant such as water is fed to the saw blade to lubricate, remove material, contain dust and cool the blade.
Current slab saws typically use a belt and pulley system to drive the saw blade. A drive pulley is placed directly on the crankshaft or fly wheel of the motor. Power is transferred to the blade through a series of subsequent belts and pulleys. It is preferable to rotate the saw blade at a particular number of revolutions per minute (RPM) while retaining as much horsepower (HP) and torque as possible. The desired RPM for a given saw blade is primarily a function of its diameter. Smaller diameter blades are spun faster than larger diameter blades. A rule of thumb is to achieve 10,000 inches of blade rotation per foot of cut. Thus a small diameter blade spins faster (at a higher RPM) to achieve the 10,000 inches of blade rotation per foot of cut at the same linear cutting pace than does a larger diameter blade.
Current slab saws require changing the RPM of the motor, and thereby the horsepower and torque output of the motor, in order to change the RPM of the saw blade. With the RPM change, the reciprocating engines of current art suffer from sometimes dramatic drops in torque output. The alternative is to change the size of the various pulleys in the drive system which is cumbersome and time and labor intensive, and while this can sometimes be accomplished, the necessary sheave sizes can require the reciprocating engine saw to be even larger and more cumbersome. The manipulation of the motor RPM can cause the motor to be operated outside of its operating specifications thus shortening its life and utility. As an example, the industrial version of a V6 motor has a maximum RPM of approximately 5,000 RPM and is recommended to be operated at approximately 2,750 RPM. With a typical one-to-one pulley set up, such a motor can run a saw blade having a recommended operating speed of 2,750 RPM at its recommended RPM. However, use of a smaller diameter blade requiring a higher blade RPM could require increasing the RPM's of the motor to its limit. On the other hand, the use of a larger diameter saw blade would require lowering the RPM of the motor such that the motor will not function well enough to deliver constant horsepower and torque to the saw blade. Consequently, with current slab saws either the motor or the saw blade are often used outside of its recommended range of RPM in order to accommodate the use of different diameter saw blades with the motors currently in use. The steep torque curve of reciprocating engines allows for only limited sheave changes. Versions of the V6 saw, where some changing of the sheaves or drive belts has been incorporated to manipulate RPM, rendered it to be even larger and further reduced the circumstances for which it is of suitable size.
Slab saws can be categorized in a fairly straightforward manner: small, large, and specialty. The utility of a small saw is generally not constrained by its size but rather its power in general commercial use. It is used where access or space limits do not allow a larger saw, for example egress via a 30 inch wide door. A slab saw using an 18HP Honda engine would be an example of this category. Large saws conversely generate substantially higher power, but are constrained by their size in many instances. A saw using a 78HP Deutz diesel would be an example of this category. However, these saws generally do not fit through even 36 inch wide doors. Specialty saws are generally so large or of unique configuration such that their application is generally to a narrow range of functions, such as a deep cut saw for cutting runways at airports for example. It is uncommon for any of the above saws to be configured to use a blade smaller than an 18 inch diameter and larger than a 54 inch diameter with the vast majority of blade use between these two extremes.
Slab saws also tend to veer away from the line of cut during operation as a result of the rotation of the saw blade acting as a drive wheel. Consequently, operation of the slab saw requires the operator to apply significant controlling pressure to the slab saw in order to properly steer the slab saw. A sufficient amount of steering force may be required to be applied to the slab saw by the operator resulting in strain and fatigue of the operator. The amount of steering force required is a direct function of the horsepower and torque of the saw.